1. Field of the Invention
The present invention relates to systems and methods for imaging distant or near sources of gamma rays using the Compton Effect.
2. Related Art
Systems and methods for imaging distant or nearby sources of gamma rays using the Compton Effect are commonly referred to as a Compton telescope. A Compton telescope typically comprises one or many gamma ray detectors combined with electronics to determine the direction and energy of gamma rays incident on the telescope. Since gamma rays are not easily focused by refractive or reflective optics in the manner of visible light and lower energy photons, existing Compton telescopes usually do not rely on focusing optics to form images, but instead use the physics of Compton scattering and multiple particle interactions within the telescope to determine the energy and momentum (and hence direction) of incident gamma rays.
Gamma ray detectors may be classified into several types according to their composition and principles of operation. The different types vary widely in their cost, available size, and detection capabilities. These classifications may include Gas Ionization detectors, Organic scintillators, Inorganic scintillators and Semiconductor Gamma Ray Detectors. Each type is discussed briefly below.
Gas Ionization detectors such as the well-known Geiger-Müller Tube or Geiger Counter produce a pulse of electric current when a gamma ray or other energetic particle ionizes an inert gas in a high voltage chamber. Low-cost gas ionization detectors (<$200 (USD) each) such as the Geiger Counter count gamma rays above a certain energy threshold but cannot measure the energy of the gamma ray. More expensive Geiger-Müller proportional ionization detectors can measure the energy deposited by a gamma ray. Very expensive (>$100,000) multi-wire proportional gas ionization chambers can measure the energy and also track the momentum of charged particles that recoil from multiple gamma ray collisions, allowing imaging of gamma ray sources.
Organic scintillators are typically solid organic polymers (plastics) like polyvinyl toluene (PVT) or liquid organic solvents like benzene containing fluorescent organic compounds (fluors) like 2,5-diphenyloxazole (PPO). When a gamma ray interacts with a scintillator, it deposits energy that excites nearby fluors. The fluors emit visible light proportional to the amount of energy deposited, and this visible light can be measured with photomultiplier tubes (PMTs) or photodiodes. Typical plastic scintillators in bulk quantities cost less than $80 per kilogram (1 kg PVT is about 1 Liter volume), while liquid scintillators may be an order of magnitude less expensive per unit volume. (David C. Stromswold, Edward R. Siciliano, John E. Schweppe, James H. Ely, Brian D. Milbrath, Richard T. Kouzes, and Bruce D. Geelhood, “A Comparison of Plastic and NaI(Tl) Scintillators for Vehicle Portal Monitor Applications,” IEEE Nuclear Science Symposium Conference Record 2003, Vol. 2, p. 1065 (2003).). For large volume particle detectors, these materials are among the least expensive known. Organic scintillators are usually used for counting gamma rays but not for measuring their energy, because the low density and low nuclear charge Z of these organic materials result in poor capture efficiency: a gamma ray with energy over 100 keV will usually Compton scatter out of an organic detector several cm in size, depositing some but not all of its energy.
Inorganic scintillators are typically fluorescing salt or oxide crystals of much higher density and higher nuclear charge Z than organic scintillators. The most common and least expensive is thallium-doped sodium iodide or NaI(Tl). Inorganic scintillators have much higher gamma ray capture efficiency than organic scintillators, and are often used to measure the energy of gamma rays in the range 10 keV to 3 MeV for laboratory, research, safety, environmental monitoring, minerals exploration, and security purposes. A typical block of inorganic scintillator can measure the energy of gamma rays between 500 keV and 3 MeV with 3% to 7% energy resolution. As a common laboratory example, a 7.5 cm diameter×7.5 cm long cylinder of NaI(Tl) has about 30% efficiency in capturing the full energy of incident mono-energetic 2.2 MeV gamma rays from the nuclear reaction n+p→2H+2.2 MeV γ. An energy spectrum of this gamma ray source in such a NaI(Tl) detector would show a peak at 2.2 MeV with a 5% full-width-at-half-maximum (FWHM) resolution. Sodium iodide scintillators in bulk quantities currently cost at least $2500 per cubic decimeter (Liter volume), while other inorganic scintillators with higher capture efficiency and better energy resolution cost from 3× to 10× as much per unit volume.
Semiconductor Gamma Ray Detectors are based on doped silicon, germanium, and similar semiconductors, placed across high voltage electrodes and often cooled to cryogenic temperatures for better performance. When a gamma-ray interaction excites electrons from the valence band to the conduction band in these materials, a conduction current flows between the electrodes, giving a very accurate measurement of the energy deposited by the gamma ray. Detectors of this type usually have higher capture efficiency per unit volume than organic scintillators but lower capture efficiency than inorganic scintillators. Semiconductor detectors have the best energy resolution of all standard particle detectors, able to measure 100 keV to 3 MeV events with better than 1% energy resolution and in some cases better than 0.1% FWHM. Semiconductor detectors also have the highest cost per unit volume, currently over $100,000 per cubic decimeter, although semiconductor detectors greater than a few hundred cubic cm are never in practice manufactured as a single detector element, but typically as a segmented array of detector strips or blocks for particle tracking.
Compton telescopes have been designed, built, and operated for astrophysical observations. These include the CompTel Gamma Ray Telescope, launched in 1991 on the Compton Gamma Ray Observatory satellite (CGRO). CompTel used a two-layer Compton telescope design consisting of organic Ne-213A liquid scintillator cells in the first layer and NaI(Tl) inorganic scintillator blocks in the second layer. According to published statistics, CompTel has an upper layer active area of 4188 cm2 with a gamma ray captures efficiency from 1.2% to 0.5% for gamma rays in the energy range 0.8 MeV to 30 MeV, with an energy resolution of 5% to 8% FWHM and angular resolution 2° to 4° for gamma rays over this energy range. Since CompTel was launched, the United States Naval Research Lab (NRL) along with many partner institutions has proposed several successor Compton telescope instruments including the ATHENA concept and the ACT concept. These proposed designs achieve higher performance than CompTel but require higher component costs, using arrays of semiconductor detectors typically in conjunction with scintillators to achieve higher gamma ray capture efficiency, better energy and angular resolution than CompTel, and in some cases a wider range of gamma ray energies.
NRL developed a significant theoretical breakthrough with the 3-Compton Principle. R. A. Kroeger, W. N. Johnson, et al, “Three-Compton Telescope: Theory, Simulations, and Performance,” IEEE Trans. Nucl. Science, Vol. 49, No. 4, p. 1887 (2002). See also U.S. Pat. No. 6,528,759, to Kurfess et al, issued Mar. 4, 2003. This discovery shows how the energy and direction vector of an incident gamma ray can be recovered in a Compton telescope if the gamma ray Compton-scatters 3 times or more inside the device. Prior to this discovery, CompTel and other early Compton telescopes only processed gamma ray events that interacted in exactly two detector layers. The 3-Compton Principle allows Compton telescopes of much higher capture efficiency, since an arbitrary number of layers of detector material may be used.
Previous large-area Compton telescopes have existed only as one-of-a-kind research instruments, with many hand-assembled components built by graduate students, laboratory engineers, and post-doctoral scientists, usually for astrophysical research applications. As a result, these large Compton instruments typically cost many millions of (US) dollars. Smaller Compton-scatter imaging systems have been built for medical research applications, but these generally use very expensive cryogenic semiconductor elements that cannot be scaled up cost-effectively to large collection areas.
There are several current applications that would benefit from a Compton telescope with a large collection area (from several square feet up to several square meters), high gamma ray capture efficiency, modest cost, and modest energy and angular resolution requirements. For example, in the field of Homeland Security, a large area Compton telescope would be useful for monitoring points of national entry and major urban centers to search for smuggled nuclear weapons, radiological dirty bombs, or Special Nuclear Materials (SNMs). However, the exceptionally high cost of existing Compton telescope designs renders this technology outside the realm of practical reality.
Another likely application is in the detection of large concealed explosives such as the roadside Improvised Explosive Devices (IEDs) used by insurgents in conflict areas to attack convoys of vehicles, or concealed vehicle-born explosives moving through a security checkpoint toward a sensitive target such as a federal building, landmark skyscraper, bridge, or crowded stadium. While conventional explosives do not emit gamma rays, they can be identified by the method of Prompt-Gamma Neutron Activation Analysis (PGNAA), using a neutron source to probe a suspected target and a gamma ray detector to analyze the element-specific gamma rays emitted by the material. This technique is described in detail in U.S. Pat. No. 7,573,044 to Norris, issued Aug. 11, 2009, the entire disclosure of which is hereby incorporated by reference.
In the past, PGNAA has been used successfully to identify concealed explosives inside metal containers at ranges up to about 50 cm from a neutron source and gamma detector. To make a useful IED or vehicle-born explosive detector, the effective PGNAA range must be extended to at least several meters, and this requires a large area gamma ray detector with imaging capability to distinguish threatening concentrations of nitrogen from harmless background concentrations such as are found in ambient air.
As yet another exemplary application, it has been estimated that there are billions of (US) dollars of recoverable metals in mining tailings around the world. Currently, the cost of assaying very large fields of mining tailings is often high enough to prevent the recovery of much of this metal, since the process requires samples to be collected from survey locations, analyzed chemically in a laboratory for minerals of interest, and then after a delay, the mineral content of tailings zones is reconstructed from the lab results. PGNAA with a range of several meters could allow mineral concentrations for large volumes of tailings to be analyzed more promptly on site, making minerals recovery from tailings more profitable.
Accordingly, there is a need in the art for a low cost, large-collection-area Compton telescope. However, one perceived obstacle to the prospect of low cost, large-collection-area Compton telescopes has been the assumption, in the gamma ray detector literature, that organic scintillators cannot measure particle energy accurately enough. As H. H. Vo and colleagues at the University of Osaka have recently shown (H. H. Vo, S. Kanamaru, C. Marquet, H. Nakamura, M. Nomachi, F. Piquemal, J. S. Ricol, Y. Sugaya, and K. Yasuda, “Energy Resolution of Plastic Scintillation Detector for Beta Rays”, IEEE Trans. Nucl. Science, Vol. 55, No. 6, p. 3723, 2008), plastic scintillators can indeed measure particle energy accurately enough, but not when coupled to a single photomultiplier tube (PMT) as in the standard apparatus in the art of gamma ray detection. Because plastic scintillators have low gamma ray stopping power and imperfect optical transparency, they do not produce a uniform optical signal from everywhere within the volume of a single organic scintillator element large enough to capture a statistically significant fraction of energetic gamma rays. H. H. Vo and colleagues showed that by coupling multiple photodetectors to a single large plastic scintillator element, the sum of the signals of the multiple photodetectors could produce a consistent energy measurement with adequate energy resolution (7% to 4% resolution for particles in the energy range from 700 keV to 1700 keV).